A pin is an incredibly powerful tactical motif that occurs when an defending piece is restricted from moving because doing so would expose a more valuable piece behind it to capture by a long-range attacker (bishop, rook, or queen). Pins are categorized as absolute (when the piece shields the king, making its movement illegal) or relative (when movement is legal but strategically disastrous, exposing a queen or rook).
A pinned piece loses its tactical efficiency and becomes highly vulnerable to attacks. A primary rule of thumb in chess tactics is: 'Exert maximum pressure on the pinned piece by attacking it with pawns or less valuable units.' Practicing pin puzzles teaches you to restrict your opponent's coordination and break down their defenses.